Safe Operating Envelope

Just as there are safe operating envelopes for aircraft, there are safe operating envelopes for pushing and pulling carts.

“The ‘envelope’ was a flight-test term referring to the limits of a particular aircraft’s performance,how tight a turn it could make at such-and-such a speed, and so on.” The Right Stuff, by Tom Wolfe

Test pilots push themselves and their aircraft to extremes to determine safe operating envelopes for that aircraft. In doing so, they work out the limits of human and mechanical abilities in all the various situations expected while flying the aircraft. A flight test program that only looked at straight and level flight wouldn’t be a good estimate of a safe operating envelope for landing an aircraft. An important component of that testing is understanding the limits of a pilot’s ability to withstand the stresses placed upon him or her while maneuvering the plane; for example, a too high speed or too tight turn may cause the pilot to lose consciousness and crash.

Just as there are safe operating envelopes for aircraft, there are safe operating envelopes for pushing and pulling carts. Just as for aircraft, the strength and endurance capabilities of the human operator play an important role in determining the safe operating envelopes for carts. Operating forces that stress the operator beyond their limits are outside the safe operating envelope. Consequently, measuring the forces exerted by the cart operator and comparing those forces to known operator limits is the basis for determining a safe operating envelope.

Tools such as the Liberty Mutual Tables (Snook tables) describe the limits to forces for starting a cart moving or for keeping it moving. Depending on who is moving the cart (men or women), how far the cart is moved and how frequently the cart is moved, the tables tell us what percentage of people can work safely within that operating envelope.

However, the forces measured on the job and compared to those in the tables must be operating forces used when moving carts with typical floor conditions, slopes, space constraints, temperature, etc. Emergency conditions must be considered too; a heavily-loaded rolling cart has a lot of momentum. Overcoming that momentum while stopping or turning that cart quickly will likely require the operator to exert a much greater force than is needed to start or keep the cart moving.

Determining the safe operating envelope for carts cannot be done by limiting operating force assessments to a straight and level flight in optimal conditions, e.g. a brand-new cart rolling on a smooth surface. The essence of developing a complete safe operating envelope for moving carts is evaluating the human performance limits in all the various situations and conditions expected while moving the cart.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.